Antioxidant system for polyolefins



ANTIOXIDANT SYSTEM FOR POLYOLEFINS Filed Aug. 5, 1966 2 Sheets-Sheet 1O: FOUND EMBRITTLEMENT TIME HOURS 0 l l l l I l I l o 1o so so so I00 AI00 90 e0 70 so so 40 30 20 no 0 B RATIO F /G. nvvszvron K. R. MILLS A TTORNEYS Nov. 5, 1968 K. R. MILLS ANTIOXIDANT SYSTEM FOR POLYOLEFINS 2Sheets-Sheet 2 Filed Aug.

O O O WEDOI m2; .PZwEMJFFEQEM RATIO INVENTOR K. R. MILLS ATTORNEYS l l ll l I i United States Patent 7 ABSTRACT OF THE DISCLOSURE A polymerstabilizer system comprising an organic phosphite compound such asdioctyl phosphite; a thiol ester such as dilaurylthiodipropionate,2,6-di-tert-butyl-4- methylphenol; and one of1,1,3-tris(2-methyl-4-hydroxy- S-tert-butylphenyl)butane and tetrakis[3-(3,5-di-tertbutyl-4-hydroxyphenyl propionyloxymethyl] methane.

This is a continuation-in-part application of an application bearingSer. No. 516,349, filed Dec. 27, 1965 now abandoned.

This invention relates to a stibilizer combination useful in thestabilization of polymers of l-olefins having 2 to 8 carbon atomsagainst deterioration in physical properties as a result of exposure tolight and air, particularly at elevated temperatures and over longperiods of time. One aspect of this invention relates to a stibilizercomposition useful in stabilizing polypropylene compositions.

It is to be understood that the term polymers of 1- olefins usedhereinafter in the specification and the claims includes copolymers,terpolymers and quaternary polymers of these l-olefins. These polymerscan also contain not more than 8 to 10 percent of other copolymerizablemonomers. V

Polypropylene is a tough, high-melting polymeric material, but inseveral respects its stability leaves much to be desired. The polymershows a tendency to decrease rapidly in melt viscosity and then tobecome brittle when kept at elevated temperatures for the time requiredin milling and calendering, or in extruding, injection molding, or fiberforming equipment. This deterioration is particularly serious when thepolymer is worked in the molten state in the presence of oxygen, e.g.air. Shaped polymers prepared in such equipment show a tendency todiscolor, to crack, and to powder around the edges upon exposure tosunlight, When the polymer or shaped polymer is heated at elevatedtemperatures, really an accelerated aging process, the problem isespecially accentuated.

To meet commercial requirements, it is of course quite important thatthe polymer retain its physical properties during processing andthereafter. However, the art has experienced great difficulty inachieving the necessary stabilization. In all probability the changesleading to this deterioration in physical properties arise from chemicalmodification of the polymer. Whether this modification is due tooxidation or to some other effect is not yet known. Lack of suchknowledge has naturally hindered the development of satisfactorystabilizers,

Considerable research over the past few years to solve these problems ofdeterioration has led to the suggestion of a variety of stabilizers tocounteract one or more of these diificulties. Most polypropylene on themarket today contains one of these stabilizers. However, no stabilizeror combination of stabilizers has yet been disclosed which is capable ofcoping with all of the stabilization problems. While it is now possibleto stabilize polypropylene fairly well against deterioration in meltviscosity, this stabilization is not always accompanied by an inhibitionof deterioration in other respects, such as embrittlement and 3,409,587Patented Nov. 5, 1968 discoloration at elevated temperatures.Stabilizers which can cope with embrittlement are not capable ofpreventing discoloration, or reduction in melt viscosity. In additionthe retention of the polymers physical properties over long periods oftime is particularly difficult to achieve.

Logically, it would seem that if one stabilizer does not do the entirejob, a combination should. Groups of stabilizers, called stabilizersystems have been proposed; however, the effect of a plurality ofstabilizers is impossible to predict from their effects individually,because the possible eflects multiply with the number of stabilizers inthe system. A stabilizer to be effective at all must be a reactivesubstance and stabilizers can react with each other as well as withpolypropylene and with whatever leads to the deterioration. The variousstabilizers thus can and do introduce unpredictable and undesirable sideeffects by reactions between themselves, and stabilizers which arecapable of improving resistance to deterioration in some ways whencombined, may actually offset that individual effect and increase therate of deterioration in that or in other respects. This may be due toone stabilizer effecting the desired improvement, another stabilizerreacting with and removing the first, and the reaction productsaccelerating the deterioration. Under such circumstances, developing astabilizer system which is capable of producing a polypropylene that hasa good overall resistance to all of the deteriorative tendencies isobviously quite a complex problem.

In accordance with this instant invention, a stabilizer system isprovided which not only improves the resistance of the polymer todiscoloration but also improves the resistance of the polymer toembrittlement upon exposure to air and light at normal atmospheric andat elevated temperatures, particularly over long periods of time.

In a copending application, Ser. No. 266,012, assigned to a commonassignee, Phillips Petroleum Company, now US. Patent 3,349,058, issuedOct. 24, 19 67 and in which the present applicant, Kenneth R. Mills, isthe coapplicant along with Arthur A. Harban, there is disclosed astabilizer system which comprises a combination of three stabilizers. aphenolic material, an organic phosphite and a thioester. This stabilizersystem prevents the formation of voids in films or filaments of apolymer of a monoolefin.

It has now been surprisingly discovered that if a stabilizer systemcomprises an organic phosphite compound, a thioester, a specificphenolic material and a second specific phenolic material, that thissystem has a stabilizing action distinctly superior to that which could'be expected from the sum of the actions of the various components takenalone. The employment of the two phenolic compounds in combinationproduces this unexpected result.

Therefore, it is an object of this invention to provide a novelstabilizer system comprising an organic phosphite, a thioester,2,6-di-tert-butyl-4-methylphenol and a second specific phenolic compoundselected from the group consisting of 1,1,3-tris2-methyl-4-hydroxy-S-tert-butylphenyl)butane and tetr-akis[3 (3,5di-tert-butyl-4-hydroxyphenyl propionyloxymethyl] methane.

Another object of this invention is to provide a stabilized polymer of al-olefin having from 2 to 8 carbon atoms by incorporating therewith astabilizer system comprising dioctyl phosphite,dilaurylthiodipropionate, 2,6-ditert-butyl-4-methylphenol and a secondspecific phenolic compound selected from the group consisting of1,1,3-tris (2 methyl-4-hydroxy-5-tert-butylphenyl)butane and thetetrakis[3 (3,5 di tert butyl 4 hydroxyphenyl)propionyloxymethyl]methane.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the'sarne' becomes better understood byreference to the following detailed description which is considered inconnection with the accompanying drawing wherein:

FIGURE 1 graphically illustrates the synergistic efiect obtained instabilizing a l-olefin polymer against embrittlement by using as thesecond specific phenolic compound, 1,1,3tris(2-methyl-4-hydroxy-5-tert-butylphenyl)-butane designated by theletter B;

FIGURE 2 graphically illustrates the synergistic effect obtained instabilizing a l-olefin polymer against v em; brittlement by using as thesecond specific phenolic c0mpound tetrakis 3- 3,5-di-tert-butyl-4-hydroxyphenyl prop'ionyloxymethyHmethane designated bythe letter C. I

Broadly this invention comprises a process for stabilizing a polymer ofa l-olefin containing from 2 to 8 carbon atoms against deterioration,said process comprising the step of contacting said polymer with astabilizing system comprising a phosphite ester characterized by thestructural-formula:

wherein R is a hydrocarbon radical selected from the group consisting"of alkyl, aryl, cycloalkyl, and combinations thereof such as alkaryland aralkyl, having 1 to 20 carbon atoms, and R and R" are selected fromthe group consisting of R and hydrogen; a thioester characterized by thestructural formula:

wherein R'" and R"" are alkyl groups containing 6 to 24 carbon atoms,and n and m are integers from 1 to 6, preferably 2;2,6di-tert-butyl-4-methy1phenol and a second specific phenol selectedfrom the group consisting of 1,1,3tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane and tetrakis[3 (3,5di-tert-butyl-4-hydroxyphenyl)propionyloxymethyl] methane.

Exemplary organic phosphite esters are: monomethyl phosphite, trimethylphosphite, trieicosyl phosphite, di-2- ethylhexyl phosphite, diphenyl2-ethylhexyl phosphite, dibutyl phosphite, di-isooctyl tolyl phosphite,tri-2-ethylhexyl phosphite, phenyl dicyclohexyl phosphite, phenyldiethyl phosphite, triphenyl phosphite, octyl phosphite, isobutylphosphite, tricresyl'phosphite, tri(2,3-dimethylphenyl) phosphite,trioctadecyl phosphite, phenyl phosphite, dioctyl phosphite (DOPI),triisoctyl phosphite, tri dodecyl phosphite, isooctyl diphenylphosphite, diisooctyl phenyl phosphite, tri(2-octylphenyl) phosphite,tri(3- nonylphenyl) phosphite, dicyclohexyl phosphite, benzyl methylisopropyl phosphite, butyl dicresyl phosphite, isooctyldi(2-octylphenyl) phosphite, di(2-ethylhexyl) 3- isooctylphenylphosphite, tri(2-cyclohexylphenyl) phosphite, tri 2 naphthyl phosphite,tri(3 phenylphenyl) phosphite, tri(Z-phenylethyl) phosphite, tridodecylphosphite, tri-4-tert-butylphenyl phosphite, dodecyl diphenyl phosphiteand 4-tert-butylphenyl di-Z-ethylhexyl phosphite I I i .Exemplarythioester compounds are: laurylhexylthio= dipropionate,dilaurylthiodiproprionate (DLTDP),- butylstearylthiodipropionate,2-ethylhexyllaurylthiodipropionate, di-Z-ethylhexylthiodipropionate,'diisodecylthiodiproheptadecylthiodiheptanoate, the '1-lauryl-8-stearyldiester pionate, isodecyltetradecylthiodiheptanoate,laurylstearylthiodipropionate, distearylthiodipropionate,hexyltetracosylthiodiacetate, octyltetradecylthiodibutyrate, heptylof4-thiaoctanedioic acid, the l-hexyl-lO-tetracosyl diester of3-thiadecanedioic acid, and the like.

A suflicient amount of the stabilizer combination is used to improve thestability of the polymer against deterioration in physical properties,including discoloration and embrittlement, under the conditions to whichthe polymer, i.ei polypropylene, will-be subjected. Very small amountsare usually adequate. Amounts within theirange from about 0.15 .to 3.50,preferably 0.5 to 2.5 parts by weight of the'stabilizer composition per100 parts of polymer (p.h.p.) are employed for optimum stabilization.

Preferably, the stabilizer system comprises from about 0.025 to about0.5 p.h.p. each of 2,6-di-tert-butyl-4-methylphenol and eitherl,1,3-tris(2-methyl-4-hydroxy-5-tertbutylphenylybutane or tetrakis[3(3,5-di-tert-butyl-4-hydroxyphenyl)propionyloxymethyl]methane, fromabout 0.05 to about 1.25 p.h.p. of the phosphite ester, and from about0.05 to about 1.25. p.h.p. of the thioester. v,

The stabilizer system of the subject invention can be formulated as asimple mixture for incorporation in the polymer. An inert organicsolvent can be used to facilitate handling, if the ingredients do notform a homogeneous mixture or solution. Incorporation into the polymercan be by simple blending, by spraying of a solution of the stabilizerson the polymer followed by drying, or by other means known to theindustry.

The poly-l-olefines with which the stabilizer system of the inventioncan be used are any of those known to the industry, andcan be made, forexample, by the so-called high-pressure process, by the low-pressureprocess of U8. 2,825,721, or by the use of any of the known organometalcatalyst systems. Examples of other copolymerizable monomers which canbe present in the copolymer are butadiene, vinyl acetate, isoprene andthe like. It is also within the scope of the invention to add othermaterials to the mixture of polyolefin and stabilizer e.g., plasticizingagents, lubricants, dyes, fillers, pigments, antistatic agents, and thelike.

The stabilizing effect of this stabilizer systemv on polypropylene wasevaluated in the following examples and is graphically demonstrated inFIGURES 1 and 2 wherein FIGURE 1 is based on the data of Example I andFIGURE 2 is based on the data of Example II.

By examination of the straight line connecting the embrittlement timesfor runs 1 and 5 in FIGURE 1, it will be seen that for any concentrationthere is a synergistic improvement obtained by the use of2,6-di-tert-buty1-4- methylphenol (A) and1,1,3-tris(2-methyl-4-hydroxy-5- tert-butylphenyl)butane (B) incombination with a phosphite ester and a thioester. Similarly, byexamination of the straight line connecting the embrittlement times forruns 6 and 10 in FIGURE 2, it will be seen that for any concentrationthere is a synergistic improvement obtained by the use of2,6-di-tert-butyl-4-methylphen0l (A) and tetrakis[3(3,5-di-tert-butyl-4-hydroxyphenyl)propionyloxymethyl]methane (C) incombination with a phosphite ester and a thioester. The designations ofA, B and C are employed because the number of letters in the namesof thephenolic components makes it difiicult to represent on the drawing.

Example I Polypropylene prepared by mass polymerization of propylene inthe presence of a catalyst system comprising diethylaluminum chlorideand the reaction product of .titaniumtetrachloride and aluminum havingthe approximate formula TiCl 1/ 3 A101 was treated with thefourcomponent stabilizer system of this invention, and withthree-component systems using only one of -the phenolic antioxidants.The stabilizer components were dissolved in acetone and this solutionwas mixed with polypropylene fluff to form a homogeneous slurry. Thesolvent was evaporated and the polypropylene was compression-molded intofilms 20 mils thick.

P.h.p. Added to Polypropylene Embrittlement Time, hr. b

(B) (A) iFouud Expected All samples also contained 0.9 p.h.p.dilaurylthiodipropionate and 0.1 p.h.p.

dioctyl phosphite. P.h.p., parts by weight per 100 parts of polymer.

b Five specimens were cut from a compression-molded film 20 mils thickand were aged in an air circulating oven at 150 C. These specimens werechecked periodically for failure as indicated by the appearance of spotgranulation, usually accompanied by discoloration. The average failuretime for the five samples was reported as the embrittlement time.

EXAMPLE i1 Another sample of polypropylene as prepared inEx ample I wastested with a stabilizing system which included as the phenolicconstituents 2,6-di-tert-butyl- 4 me'thylphenol (A) andtetrakis[3-(3,S-di-tert-butyl-4-hydroxyphenyl)propionyloxymethyl]methane(C):

4. A composition having improved resistance to discoloration andembrittlement on aging and heating comprising a polymerforrned frommonomers selected" from the group consisting of l-olefins havihg'2 to8'carbon atoms in combination with a stabilzer system comprising P.h.p.Added to Polypropylene Embrittlement Time, hours (A) Found Expected Allsamples also contained 0.4 p.h.p. dilaurylthiodipropionate and 0.05p.h.p.

dioctyl phosphite (p.h.p.=parts by weight per 100 parts of polymer).

wherein R is a hydrocarbon radical selected from the group consisting ofalkyl, aryl, cycloalkyl, alkaryl and aralkyl having 1 to carbon atoms,and R and R" are selected from the group consisting of R and hydrogen;(2) a thioester characterized by the formula:

wherein R and R"" are alkyl groups selected from those containing 6 to24 carbon atoms, and n and m are integers from 1 to 6; (3)2,6-di-tert-butyl-4-methylphenol; and (4) tetrakis[3 (3,5di-tert-butyl-4-hydroxyphenyl) propionyloxymethyl]methane.

2. A stabilizer system in accordance with claim 1 wherein (3) and (4)are each present in an amount which provides in the polymer a range offrom about 0.025 to about 0.5 part by weight per 100 parts of polymer,(1) is present in an amount which provides in the polymer a range offrom about 0.05 to about 1.25 parts by weight per 100 parts of polymer,and (2) is present in an amount which provides in the polymer a range offrom about 0.05 to about 1.25 parts by weight per 100 parts of polymer.

3. A stabilizer system in accordance with claim 2 in which (1) isdioctyl phosphite and (2) is dilaurylthiodipropionate. a

(1) an organic phosphite ester characterized by the following formula:

0R ROP/ wherein R is a hydrocarbon radical selected from the groupconsisting of alkyl, aryl, cycloalkyl, alkaryl and aralkyl having 1 to20 carbon atoms, and R and R" are selected from the group consisting ofR and hydrogen; (2) a thioester characterized by the formula:

CnHanOOOR' I cums-o o 0 a wherein R' and R" are alkyl groups selectedfrom those containing 6 to 24 carbon atoms, and n and m are integersfrom 1 to 6; (3) 2,6-di-tert-butyl-4-methylphenol; and (4)tetrakis[3-(3,5 di-tert-butyl 4 hydroxyphenyl)propionyloxymethyl]methane.

5. A composition according to claim 4 wherein the polymer ispolypropylene, (3) and (4) are each present in a range of from about0.025 to about 0.5 parts by weight per 100 parts of polymer, (1) ispresent in the range of from about 0.05 to about 1.25 parts by weightper 100 parts of polymer, and (2) is present in the range of from about0.05 to about 1.25 parts by weight per 100 parts of polymer.

6. A composition according to claim 5 wherein (1) is dioctyl phosphiteand (2) is dilaurylthiodipropionate and said stabilizer system ispresent in the range of from about 0.15 to about 3.50 parts by weight ofthe stabilizer system per 100 parts of polypropylene.

7. A stabilizer system for use in improving the resistence ofpolypropylene to deterioration in physical properties on exposure tolight and heat, comprising (1) dioctyl phosphite; (2)dilaurylthiodipropionate; (3) 2,6-di-tertbutyl-4-methylphenol; and (4)1,l,3-tris(2-methyl-4-hydroxy-S-tert-butylphenyl)butane.

8. A stabilizer system according to claim 7 wherein (3) and (4) are eachpresent in an amount which provides in the polymer a range of from about0.025 to about 0.5 part by weight per 100 parts of polymer, (1) is pres-7 ent in an amount which provides in the polymer a range of from about0.05 to about 1.25 parts by weight per 100 parts of polymer, and (2) ispresent in an amount which provides in the polymer a range of from about0.05 to about 1.25 parts by weight per 100 parts of polymer.

9. A composition having improved resistance to discoloration andem'brittlement on aging and heating comprising polypropylene incombination with a stabilizer system comprising (1) dioctyl phosphite;(2) dilaurylthiodipropionate; (3) 2,6 di-tert-butyl-4-methylphenol; and(4) 1,l,3-tris(2 methyl 4 hydroxy-S-tert-butylphenyl) butane.

10. A composition according to claim 9 wherein (3) and (4) are eachpresent in the range of from about 0.025 to about 0.5 part by weight per100 parts of polymer, (1) is present in the range of from about 0.05 toabout 1.25 parts by weight per 100 parts of polymer, and (2) is presentin the range of from about 0.05 to about 1.25 parts by weight per 100parts of polymer, and said stabilizer system is present in the range offrom about 0.15 to about 3.50 parts by weight of the stabilizer systemper 100 parts of polypropylene.

References Cited FOREIGN PATENTS 638,674 3/1962 Canada.

OTHER REFERENCES Technical Bulletin 193 Topanol CA Apr. 23, 1963, I.C.IOrganics, Inc. Imperial Chemical Industries, Limited.

DONALD E. CZAJA, Primary Examiner,

V. P. HOKE, Assistant Examiner.

